The invention relates generally to the phenomenon of corrosion and, more particularly, to the protection of metallic structures or surfaces which are subjected to corrosive conditions. Of special interest is the cathodic protection of such metallic structures or surfaces.
It is known that all metallic structures which come into contact with a liquid having the properties of an electrolyte are susceptible to the phenomenon of spontaneous corrosion. Such corrosion tends to destroy the metallic structure and, depending upon the particular corrosive conditions existing, destruction of the metallic structure may occur within a longer or shorter period of time. In many instances, however, significant damage to the metallic structure may occur within a short period of time even though destruction of the metallic structure has not yet occurred.
Some examples of metallic structures which are prone to the phenomenon of spontaneous corrosion are as follows: radiating panels embedded in floor concrete; metallic pipes embedded in or passing through masonry; and standing, fixedly mounted metallic piles. All of these metallic structures may come into contact with water or some other electrolyte. Moreover, metallic structures which are exposed to the atmosphere are also susceptible to spontaneous corrosion. For instance, eaves, gutters, motor vehicle parts, the rolling gates commonly used for locking shops, etc. are all subject to this phenomenon.
It is also well known that corrosion phenomena become intensified with increasing chemico-physical variations along the metallic structure, whether these chemico-physical variations are due to variations in the metallic surface or due to variations in the electrolyte wetting the metallic surface or due to variations in both the metallic surface and the electrolyte. The reason is that such chemico-physical variations produce electro-chemical systems and electrical fields which enhance the corrosion and permit its continued progression.
It is further known that the action of such electrochemical systems and electrical fields may be nullified by simply creating electrical fields which oppose those due to the chemico-physical variations and buck those fields. One manner of achieving this is by the use of so-called "sacrificial" anodes which go into solution in the electrolyte more readily than the metallic surfaces which are to be protected. The anodes are electrically connected to the metallic surface which is to be protected from corrosion by means of a cable and are then immersed in the electrolyte. In this manner, the anodes set up electrical fields opposing those due to the chemico-physical variations thereby protecting the metallic structure from corrosion. The protection of metallic surfaces from corrosion by using anodes which are in electrical communication with the metallic surfaces and which are immersed in the same electrolyte as the metallic surfaces may be referred to as "cathodic protection" of the metallic surfaces.
However, there are many instances where cathodic protection, when carried out in the conventional and well-known manners used heretofore, cannot be utilized. This would be the case, for example, with motor vehicle parts, particularly the lower zones of doors and fenders, radiating panels and, in general, pipes which are embedded in or pass through concrete, masonry or the like, the lower edges of rolling gates, eaves, gutters, etc. The fact that cathodic protection cannot be utilized in many instances is due to the impossibility of creating the necessary protective fields under many circumstances, one of the reasons for this being that the electrolyte is often discontinuous so that the electrolytic connection between anode and surface to be protected, which is essential for the functioning of cathodic protection, is lacking.